The present invention relates to a soldering flux and a method for bonding a semiconductor element with the soldering flux, more specifically, a soldering flux and the method for bonding a semiconductor element with the soldering flux which can increase the reliability.
Recently, as electronic devices are increasingly downsized and lightened, the flip-chip method, which mounts a semiconductor chip facedown on a circuit board, is prevalently used as a mounting method for semiconductor chips such as LSI chips, etc.
The conventional flip-chip bonding will be explained with reference to FIGS. 5A-5D and 6A-6C. FIGS. 5A-5D and 6A-6C are sectional views illustrating the conventional flip-chip bonding.
A semiconductor chip 102 with solder bump electrodes, i.e., solder bumps 100 formed on is prepared.
Then, the solder bumps 100 of the semiconductor chip 102 are brought into contact with a flux 106 printed in advance on a flat plate 104 with a squeegee to transfer the flux 106 to the solder bumps 100 (FIG. 5A). The flux 106 is made of, e.g., rosin or others.
Next, a circuit board 110 with electrodes 108 formed on is prepared. Solder bumps 112 are formed on the electrodes 108 of the circuit board 110.
Next, the solder bumps 100 of the semiconductor chip 102 and the electrodes 108 of the circuit board 110 are aligned with each other (FIG. 5B).
Then, the solder bumps 100 of the semiconductor chip 102 and the solder bumps 112 of the circuit board 110 are melted with each other by heating to reflow-bond the solder bumps 100 of the semiconductor chip 102 to the electrodes 108 of the circuit board 110 (FIG. 5C).
Next, the flux 106 remaining between the semiconductor chip and the circuit board is washed off with a solvent 114 (FIG. 5D). The flux 106 is washed off so as to ensure filling and the adhesion strength of an underfill material to be filled between the semiconductor chip 102 and the circuit board 110.
Next, the underfill material 116 of a thermosetting resin is filed between the semiconductor chip 102 and the circuit board 110 (FIGS. 6A and 6B).
Next, the underfill material 116 between the semiconductor chip 102 and the circuit board 110 is cured by heating (FIG. 6C). The underfill material 116 reduces the thermal expansion strain between the semiconductor chip 102 and the circuit board 110 and prohibits the intrusion of water to thereby prevent the corrosion of the solder bumps 100, the electrodes 108, etc.
As semiconductor chips are more integrated, the number of the pins of the semiconductor chips tends to increase. The size of the solder bumps tends to be smaller, and the pitch of the solder bumps tends to be smaller. Recently, it is becoming accordingly difficult to wash off the flux after the solder bonding. The flux residue which has not been washed off and remains between the semiconductor chip and the circuit board causes defects of the filling and decreases of the adhesion strength of the underfill material, which much decrease the reliability of the bonded parts. To perfectly wash off the flux, a long period of time of the washing step and the use of a special washing apparatus are necessary, which causes cost increase.
As a countermeasure to this, as a flux to be used in the flip-chip bonding, a nonwash-type flux, which can be used without washing for removing the flux, is required. The use of a flux of a thermosetting resin instead of rosin is being studied. The nonwash-type flux of a thermosetting resin contains, e.g., epoxy resin as the thermosetting resin, a curing agent for the epoxy resin, an activator having the property of removing oxide film, etc. to activate the bonding surface and cause the solder to exert the bonding strength, and an accelerator for accelerating the cure of the epoxy resin. The nonwash-type flux of the thermosetting resin is being generally used in the surface mount, and is designed to be perfectly cured after the solder bonding so that the flux which has been cured is not melted in reliability tests, etc.
Such nonwash-type flux is used in bonding the solder bumps of the semiconductor chip and the electrodes of the circuit board, whereby the underfill material can be filled without washing the flux.
However, in the flip-chip bonding of a below 100 μm including 100 μm gap between the semiconductor chip and the circuit board, it is difficult to use the nonwash-type flux of a thermosetting resin. The reason is as follows.
The flux which has been cured after the solder bonding form an interface with the underfill material to be filled later. Accordingly, the underfill material has the adhesion strength decreased, and the reliability of the bonded parts is decreased.
The flux which has been cured after the solder bonding further narrows the gap between the semiconductor chip and the circuit board to thereby block the flow of the underfill material, which causes defective filling of the underfill material and voids. Such defective filling and voids decrease the reliability of the bonded parts.
The related arts are disclosed in, e.g., Japanese published unexamined patent application No. 2001-7158, Japanese published unexamined patent application No. 2004-1030 and Japanese published unexamined patent application No. Hei 10-195487 (1998).